The agronomic performance of plants has typically been improved by either classical plant breeding or genetic engineering. Classical breeding typically results in the transfer of unknown nucleic acids from one plant to another. Genetic engineering techniques introduce foreign nucleic acids into the plant genome, i.e., DNA that is not from a plant or that is not from a plant that is naturally interfertile with the plant to be modified by genetic engineering. For example, genetic engineering introduces non-plant nucleic acids into a plant genome. Both classical breeding and genetic engineering strategies create plant genomes that contain undesirable and unwanted genetic material, and the resultant cross-bred or transgenic plants can exhibit unfavorable traits. The inadequacies of both strategies can prove harmful to the transgenic plants, as well as to the animals and humans who consume such products.
Conventional Breeding Relies on the Transfer of Unknown DNA
Plant breeding typically relies on the random recombination of plant chromosomes to create varieties that have new and improved characteristics. Thus, by screening large populations of progeny that result from plant crosses, breeders can identify those plants that display a desired trait, such as an increase in yield, improved vigor, enhanced resistance to diseases and insects, or greater ability to survive under drought conditions. However, classical breeding methods are laborious and time-consuming, and new varieties typically display only relatively modest improvements.
Furthermore, classical plant breeding typically results in the transfer of hundreds of unknown genes into a plant genome. It is likely that some of those transferred genes encode potentially harmful allergens, such as patatin, lectins, chitinases, proteases, thaumatin-like proteins, lipid transfer proteins, amylases, trypsin inhibitors, and seed storage proteins (Breiteneder et al., J Allergy Clin Immunol 106: 27-36).
Similarly, introgressed genes can be involved in the biosynthesis of toxins including lathyrogens, hydrazines, glucosinolates and goitrogens, cumarins, saponins, alkaloids, glycoalkaloids, biogenic amines, enzyme inhibitors, such as lectins (haemagglutinins), trypsin inhibitors, chelating substances such as phytates and oxalates, ribotoxins, antimicrobial peptides, amino acids such as beta-N-oxalylamino-L-alanine, atractyloside, oleandrine, taxol, and isoquinoline (Pokorny, Cas Lek Cesk 136: 267-70, 1997). The risk of inadvertently introducing such poisons into human and animal food supplies is further increased through efforts to “untap” the genetic diversity of wild crop relatives that have not been used before for food consumption (Hoisington et al., Proc Natl Acad Sci USA 96: 5937-43, 1999).
Although classical plant breeding can easily introduce genes involved in undesirable anti-nutritional compounds into food crops and plants, it cannot easily remove them. For instance, it took about 15 years to reduce harmful phytate levels in corn and rice by inactivating Lpa genes (Raboy, J Nutr 132: 503S-505S, 2002). The long timeframe for realizing positive results is not practical, especially since there is an urgent need for methods that more effectively and efficiently improve the quality of food crops. One example of a gene that only recently was found to be associated with the synthesis of anti-nutritional compounds is the polyphenol oxidase (PPO) gene, which oxidizes certain phenolic compounds to produce mutagenic, carcinogenic and cytotoxic agents like phenoxyl radicals and quinoid derivatives (Kagan et al., Biochemistry 33: 9651-60, 1994). The presence of multiple copies of this gene in the genome of plants such as potato makes it particularly difficult to reduce PPO activity through breeding.
Even more time is needed for the removal of anti-nutritional compounds if little or nothing is known about their genetic basis. For instance, no genes have been linked to the accumulation of high concentrations of acrylamide, a potent neurotoxin and mutagen, in some potatoes that are heated to 160° C. or higher (Tareke et al., J Agric Food Chem. 50: 4998-5006, 2002). It is therefore very difficult to efficiently develop new potato varieties that produce less acrylamide during processing using conventional breeding. Thus, there is a need to grow potatoes and other carbohydrate-rich foods, such as wheat, with reduced levels of such dangerous compounds, but without the use of unknown or foreign nucleic acids.
Other anti-nutritional compounds that can accumulate during processing and are difficult to minimize or eliminate through breeding include the Maillard-reaction products N-Nitroso-N-(3-keto-1,2-butanediol)-3′-nitrotyramine (Wang et al., Arch Toxicol 70: 10-5, 1995), and 5-hydroxymethyl-2-furfural (Janzowski et al., Food Chem Toxicol 38: 801-9, 2000). Additional Maillard reaction products that have not been well characterized are also known to display mutagenic properties (Shibamoto, Prog Clin Biol Res 304: 359-76, 1989).
It can be equally difficult to rapidly increase levels of positive nutritional compounds in food crops due to the inherent imprecision of conventional plant breeding. For instance, it would be desirable to increase levels of “resistant starch” (Topping et al., Physiol Rev 81: 1031-64, 2001) in a variety of crops. Such starch is ultimately responsible for promoting immune responses, suppressing potential pathogens, and reducing the incidence of diseases including colorectal cancer (Bird et al., Curr Issues Intest Microbiol 1: 25-37, 2000). However, the only available plants with increased levels of resistant starch are low-yielding varieties like maize mutants “amylose extender”, “dull”, and “sugary-2.” Creation of new high resistant starch sources, such as potato, would enable broader dietary incorporation of this health-promoting component.
The inability to safely manipulate the genotypes of plants often leads to the use of external chemicals to induce a desired phenotype. Despite numerous breeding programs to delay tuber sprouting, for example, no potato varieties are available commercially that can be stored for months without treatment with sprout inhibitors. The latter, such as isopropyl-N-chlorophenyl-carbamate (CIPC), is linked to acute toxicity and tumor development, and can be present in processed potato foods at concentrations between 1 mg/kg and 5 mg/kg.
Genetic Engineering Relies on the Transfer of Foreign DNA
Genetic engineering can be used to modify, produce, or remove certain traits from plants. While there has been limited progress in improving the nutritional value and health characteristics of plants, most improvements target plant traits that promote ease of crop cultivation. Thus, certain plants are resistant to the glyphosate herbicide because they contain the bacterial gene 5-enolpyruvylshikimate-3-phosphate synthase (Padgette et al., Arch Biochem Biophys. 258: 564-73, 1987). Similarly, genetic engineering has produced insect-, viral-, and fungal-resistant plant varieties (Shah et al., Trends in Biotechnology 13: 362-368, 1995; Gao et al., Nat Biotechnol. 18: 1307-10, 2000; Osusky et al., Nat Biotechnol. 18: 1162-6, 2000), but few with enhanced nutrition or health benefits.
According to standard, well-known techniques, genetic “expression cassettes,” comprising genes and regulatory elements, are inserted within the borders of Agrobacterium-isolated transfer DNAs (“T-DNAs”) and integrated into plant genomes. Thus, Agrobacterium-mediated transfer of T-DNA material typically comprises the following standard procedures: (1) in vitro recombination of genetic elements, at least one of which is of foreign origin, to produce an expression cassette for selection of transformation, (2) insertion of this expression cassette, often together with at least one other expression cassette containing foreign DNA, into a T-DNA region of a binary vector, which usually consists of several hundreds of basepairs of Agrobacterium DNA flanked by T-DNA border sequences, (3) transfer of the sequences located between the T-DNA borders, often accompanied with some or all of the additional binary vector sequences from Agrobacterium to the plant cell, and (4) selection of stably transformed plant cells. See, e.g., U.S. Pat. Nos. 4,658,082, 6,051,757, 6,258,999, 5,453,367, 5,767,368, 6,403,865, 5,629,183, 5,464,763, 6,201,169, 5,990,387, 4,693,976, 5,886,244, 5,221,623, 5,736,369, 4,940,838, 6,153,812, 6,100,447, 6,140,553, 6,051,757, 5,731,179, 5,149,645 and EP 0 120,516, EP 0 257,472, EP 0 561,082, 1,009,842A1, 0 853,675A1, 0 486,233B1, 0 554,273A1, 0 270,822A1, 0 174,166A1, and WO 01/25459.
Thus, genetic engineering methods rely on the introduction of foreign nucleic acids into the food supply. Those techniques transfer complex fusions of a few to more than 20 genetic elements isolated from viruses, bacteria, and plants, that are not indigenous to the transformed plant species. Such foreign elements include regulatory elements such as promoters and terminators, and genes that are involved in the expression of a new trait or function as markers to identify or select for transformation events. Despite the testing of foods containing foreign DNA for safety prior to regulatory approval, many consumers are concerned about the long-term effects of eating foods that express foreign proteins, which are produced by genes obtained from other, non-plant species.
One commonly used regulatory element is the 35S “super” promoter of cauliflower mosaic virus (CaMV), which is typically used in plant engineering to induce high levels of expression of transgenes to which it is directly linked. However, the 35S promoter also can enhance the expression of native genes in its vicinity (Weigel et al., Plant Physiol., 122: 1003-13, 2000). Such promoters may thus induce unpredictable alterations in the expression of endogenous genes, possibly resulting in undesirable effects such as increased alkaloid production. Preferred “strong” promoters are generally those isolated from viruses, such as rice tungro bacilliform virus, maize streak virus, cassava vein virus, mirabilis virus, peanut chlorotic streak caulimovirus, figwort mosaic virus and chlorella virus. Other frequently used promoters are cloned from bacterial species and include the promoters of the nopaline synthase and octopine synthase gene.
To obtain appropriate termination of gene translation, terminator sequences are fused to the 3′-end of transgenes and include genetic elements from the nopaline synthase and octopine synthase genes from Agrobacterium. Other genetic elements may be used to further enhance gene expression or target the expressed protein to certain cell compartments. These elements include introns to boost transgene expression and signal peptide sequences to target the foreign gene to certain cellular compartments, often derived from foreign plant species.
Certain genes involved in expression of a new trait are most frequently derived from foreign sources. If native genes are used, they are often inverted to silence the expression of that gene in transgenic plants and co-transformed with foreign DNA such as a selectable marker. The main disadvantage of this “antisense” technology is that the inverted DNA usually contains new and uncharacterized open reading frames inserted between a promoter and terminator. Thus, potato plants that were genetically modified with antisense constructs derived from the starch related gene R1 (Kossmann et al., U.S. Pat. No. 6,207,880), the L- and H-type glucan phosphorylase genes (Kawchuk et al., U.S. Pat. No. 5,998,701, 1999), the polyphenol oxidase gene (Steffens, U.S. Pat. No. 6,160,204, 2000), and genes for starch branching enzymes I and II (Schwall et al., Nature Biotechnology 18: 551-554, 2000) all potentially express new peptides consisting of at least 50 amino acids (Table 1). These new peptides may interfere with plant development and/or reduce the nutritional value of potato, and are therefore undesirable.
Conventional marker genes are incorporated into genetic constructs and used to select for transformation events. They confer either antibiotic or herbicide resistance (U.S. Pat. No. 6,174,724), a metabolic advantage (U.S. Pat. No. 5,767,378), or a morphologically abnormal phenotype (U.S. Pat. No. 5,965,791) to the transformed plant. Such markers are typically derived from bacterial sources.
Furthermore, because of the infidelity of T-DNA transfer, about 75% of transformation events in plants such as tomato, tobacco, and potato contain plasmid “backbone” sequences in addition to the T-DNA (Kononov et al., Plant J 11: 945-57, 1997). The presence of such backbone sequences is undesirable because they are foreign and typically contain origins of replication and antibiotic resistance gene markers.
There do exist various methods for removing elements like foreign marker genes, but few are easily applicable to plant genetic engineering. According to one such method, the marker gene and desired gene or nucleotide sequence are placed on different vectors. The infection of plants with either a single Agrobacterium strain carrying both vectors (U.S. Pat. No. 6,265,638) or two Agrobacterium strains each of which carries one of the vectors can occasionally result in unlinked integration events, which may be separated genetically through outbreeding. The main disadvantage of this method is that the genetic separation of loci can be very laborious and time-consuming, especially if T-DNA integration events are linked. Furthermore, this method is not widely applicable in apomictic plants, which reproduce asexually, such as Kentucky bluegrass, or vegetatively propagated crops such as potato, which cannot be readily bred due to inbreeding depression, high levels of heterozygosity, and low fertility levels.
Another method for removing foreign genetic elements relies on inserting the foreign gene, like the selectable marker, into a transposable element. The modified transposable element may then be spliced out from the genome at low frequencies. Traditional crosses with untransformed plants must then be performed to separate the transposed element from the host (U.S. Pat. No. 5,482,852). As described for the previous method, this alternative method cannot be used for vegetatively propagated or apomictic plant systems.
A third method of removing a marker gene uses the Cre/lox site-specific recombination system of bacteriophage P1 (Dale & Ow, Proc. Natl. Acad. Sci. USA, 88: 10558-62, 1991). Insertion of a marker gene together with the Cre recombinase gene and a chimeric gene involved in induction of Cre (both with their own promoters and terminators) between two lox sites leads to excision of the region delineated by the lox sites during the regeneration process (Zuo et al., Nat. Biotechnol., 19: 157-61, 2001). This complicated process is inefficient and not reliable, and may cause genome instability.
Recent studies report that some plant genes themselves may be used as transformation markers. Examples of such plant markers include Pga22 (Zuo et al., Curr Opin Biotechnol. 13: 173-80, 2002), Cki1 (Kakimoto, Science 274: 982-985, 1996) and Esr1 (Banno et al., Plant Cell 13: 2609-18, 2001). All of the genes, however, trigger cytokinin responses, which confer an undesirable phenotype to the transformed plant. Furthermore, such plant markers would still need to be removed upon transformation by any of the methods described above.
Alternative methods to transform plants are also based on the in vitro recombination of foreign genetic elements, and rely on bacterial plasmid sequences for maintenance in E. coli, parts of which are co-integrated during the transformation process. Examples of such methods to transform plants with foreign DNA are described in U.S. Pat. Nos. 5,591,616, 6,051,757, 4,945,050, 6,143,949, 4,743,548, 5,302,523, and 5,284,253.
Marker-free transgenic plants may also be obtained by omitting any selection procedures prior to regeneration. A disadvantage of this method is that most events generated through this method will represent untransformed or chimeric plants because they will usually not be derived from single transformed plant cells. It is extremely difficult and laborious to use a marker-free procedure for the identification of transgenic plants that contain the same DNA insertion(s) in all their cells.
Thus, there is a very important need to improve plants beyond that which can be accomplished through the classical breeding crosses and conventional genetic engineering techniques, and which does not rely on the insertion of unknown or foreign nucleic acid into a plant genome. Accordingly, the present invention provides methods and compositions for precisely modifying a plant's own genetic material. Thus, the inventive “precise breeding” strategy does not induce undesirable phenotypes and does not introduce unknown or foreign nucleic acid into a plant genome.